Update from trunk:
- Make ip_len and ip_off unsigned.
- Make sure we don't accept or transmit packets larger than the
maximim IP packet size.
This fixes the so-called `death ping' bug.
Sum of work from Bill Fenner <fenner@parc.xerox.com>,
Kevin Lahey <kml@nas.nasa.gov>, and myself.
Thanks to Curt Sampson, Jukka Marin, and Kevin Lahey for testing
this under NetBSD 1.2